Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric (boost pressures). A conventional turbocharger, generally designed as reference 10 in FIG. 1, essentially comprises an exhaust gas driven turbine wheel 5 within a turbine section 2 mounted on a rotatable shaft within a turbine housing 4. For instance, in a centripetal turbine the turbine housing 4 defines an annular inlet passageway around the turbine wheel and a generally cylindrical axial outlet passageway 11 extending from the turbine wheel. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing 6 of the compressor section 2. The compressor wheel delivers compressed air to the intake manifold of the engine, thereby increasing engine power.
Turbochargers incorporating wastegates are also well known, as illustrated in FIG. 1. A wastegated turbocharger has a bypass passageway between the exhaust inlet and exhaust outlet portions of the turbine housing to enable control of the turbocharger boost pressure. A wastegate valve assembly 13 is located in the passageway and is controlled to open the passageway when the pressure level of the boost air increases to a predetermined level, thus allowing some of the exhaust gas to bypass the turbine wheel preventing the boost pressure from rising further. The wastegate valve 13 is generally actuated by a pneumatic actuator 22 operated by boost air pressure delivered by the compressor wheel. The pneumatic actuator 22 may comprise a spring loaded diaphragm or sliding seal housed within a canister 24 (referred to as an actuator can) which is mounted to the compressor housing 6. The diaphragm seal acts on a connecting rod 26 which actuates the wastegate valve assembly 13. The actuator can 24 is connected to the compressor outlet via a hose to deliver boost air to the can which acts on the diaphragm (or sliding seal) to oppose the spring bias. The spring is selected, and the actuator and wastegate valve initially set, so that under low boost conditions the wastegate valve 13a remains closed. However, when the boost pressure reaches a predetermined maximum the diaphragm seal is moved against the action of the spring and operates to open the wastegate valve 13a (via the connecting actuator rod) thereby allowing some exhaust gas to bypass the turbine wheel.
In these types of conventional wastegate valves, the pressure at which the wastegate valve begins to open (the “lift off pressure”) is critical to the operation thereof. Accordingly, the wastegate valve must be very carefully set when the pneumatic actuator 22 and wastegate valve assembly 13 are assembled to the turbocharger. The precise actuator can 24 pressure, at which the diaphragm begins to move, is dependent upon the preload of the spring used. Unfortunately, the variation in the manufacturing tolerances of springs means that variations in spring rate from one spring to the next is likely, and it is necessary to calibrate each turbocharger, individually, to determine the lift off pressure.
One method of carrying out the initial set up of the conventional actuator assembly described above, is a process known as “weld to set”. The actuator can 24, actuating rod 26 and actuator lever 28 are pre-assembled, and mounted to the turbocharger 10. The wastegate valve 13a is then clamped shut from within the turbine housing 4 and the actuator can 24 is pressurized to the desired lift off pressure. With the diaphragm, actuator rod 26 and valve 13a thus held in their respective relative positions immediately prior to lift off, the end of the actuator lever is welded to the valve stem. Accordingly, any increase in the pressure supplied to the actuator above the predetermined lift off pressure will cause the valve 13a to open.
A known alternative to the above is to use an adjustable length actuator rod, typically comprising a threaded rod and rod end. The set point is achieved by adjusting the length of the rod, either by turning the rod end or a nut captured in the rod end assembly. Even using this method, it is still necessary to calibrate each turbocharger, individually, to determine the lift off pressure.
The assembly process and combined calibration process is very time intensive. Moreover, once assembled it is difficult to replace a faulty pneumatic actuator 22 and/or wastegate valve assembly 13. Conventional practice is to replace the entire turbocharger assembly, since they come pre-calibrated with an installed wastegate actuator 20 and wastegate valve assembly 13. This is an expensive replacement.